Light emitting devices and in particular semiconductor light emitting devices, such as light emitting diodes (LEDs) and vertical cavity surface emitting lasers (VCSELs), are widely used in optical communication systems and optical navigation or positioning systems, such as an optical computer mouse. Typically, these systems require the light emitting device to provide a stable, predetermined radiant output when a constant drive current or power is supplied thereto. In particular, a laser, such as a VCSEL, will only laze when the drive current is above a specified threshold, and the optical power or radiant output emitted by the laser will generally rise in predictable if not linear manner with an increase in the applied drive current. However, the radiant output of a semiconductor light emitting device can vary significantly between devices fabricated in different batches, between devices fabricated within the same batch and even between devices fabricated in or on the same substrate. Thus, following fabrication each of the devices are typically independently tested to establish the relationship between drive current or power and radiant output, and a drive circuit for the device adjusted or calibrated to provide the desired, predetermined radiant output.
Conventional methods of testing and calibrating the radiant output of semiconductor light emitting devices all suffer from one or more drawbacks or disadvantages. Referring to FIG. 1, the conventional method of parallel testing of multiple devices, such as a number n of VCSELs 12, requires a corresponding number, 1 through n, multiple reference calibration sensors 14, each precisely positioned in a platform or carriage 16 of a test system (not shown) relative to the devices under test. One shortcoming of this approach is that the reference calibration sensors 14 typically must be calibrated by an independent, outside facility to industry specified standards, i.e., NIST standards, and the test system must be disassembled and all the sensors sent out for re-calibration on a regular basis per the NIST standards. To avoid complete shutdown of the test system when the reference calibration sensors are removed for calibration at least one other complete set of calibrated optical reference sensors is required to continue the production tests. Moreover, even when another set of calibrated optical reference sensors is available, there is inevitably some delay in testing due to the need to disassemble, reassemble and verify correct alignment and operation of the sensors in the test system. Finally, like the light emitting devices themselves there is inherently and inevitably some variation in the sensitivity between the multiple calibrated optical reference sensors, which must be taken into account when determining a range of acceptable radiant outputs from the light emitting devices undergoing test. Thus, the conventional approach is not an easy, efficient nor economic method for quickly and accurately testing substantial numbers of light emitting devices.
In addition to the problems outlined above, the conventional approach is not scalable to allow parallel testing of increased numbers of light emitting devices, nor can the calibrated optical reference sensors be readily adapted for testing devices having different physical dimensions, operating at different frequencies and/or at different radiant output levels.
Accordingly, there is a need for a new test structure or system and method for quickly and accurately testing multiple light emitting devices, such as VCSELs, in parallel in an efficient and economic manner. It is further desirable that the test system and methods are scalable, and suitable for testing devices having different physical dimensions, operating frequencies and/or radiant output levels.
The present invention provides a solution to these and other problems, and offers further advantages over conventional testing structures and methods.